Electrical contact



J. 27, 1948., DURsT 2,434,992

' FIG. 2 g p $32 11 VALUABLE y METAL ALUABLE 3 ,mlm TAL I9 I? Patented Jan. 27, 1948 ELECTRICAL CONTACT George Durst, Attleboro, Mass., assignor to Metals & Controls Corporation, Attleboro, Mass., a corporation of Massachusetts Original application September 27, 1943, Serial No. 504,079, now Patent No. 2,433,687, dated Divided and this application May 13, 1946, Serial No. 669,284

December 30, 1947.

to electrical contacts composed of the more costly metals.

This application is a division of my copending application Serial No. 504,079, filed September 27, 1943, now Patent No. 2,433,687, issued December 30,1947.

Among the several objects of the invention may be noted the provision of a new form of contact which employs at the point of engagement of the contact only very little costly metal ordinarily used for the purpose; the provision of a contact of the class described in which the desired contact metal per se is exposed in definite relief and which at the same time is very strongly and rigidly supported on the remainder of the contact assembly; the provision of a process for economically making said contacts; and the provision of a construction of the class described particularly applicable to small contact structures such as used in radio, telephones, relay applications et cetera. Other objects will be in part obvious and in part pointed out hereinafter.

In the accompanying drawing, in which several of various possible embodiments of the invention are illustrated,

Figs. 1, 2 and 3 are much enlarged cross sections showing three different types of one form of my invention; and

Figs. 4, 5 and 6 respectively show the constructions of Figs. 1, 2 and 3 after a subsequent construction step.

Similar reference characters indicate corresponding parts'throughout the several views of the drawing.

Certain valuable metals and alloys form extraordinarily good electrical contacts, such as for example, platinum, platinum-iridium, goldplatinum, palladium, etc. However, due to their high cost, it is desirable to limitthe use of such metals only to the precise locations where they are needed, namely, in the case of electrical contacts where current-carrying contact is actually made. Hereinafter the metals above specified will be called valuable metals in the sense that they need to be conserved for one reason or another. The differentiating adjectives will be nonvaluable or less valuable to designate more economical metals such as iron, steel, copper, brass et cetera.

Heretofore difliculties had been encountered in reducing the amount of valuable metal used because the smaller the mass of metal becomes, the more dimcult it is to handle and to attach to the adjacent base or metal support. For example, it has been heretofore proposed electrically to weld a plain platinum wire to a metal base and to use the surface of the wire as the contact region. The difilculty with this is that it requires a size of wire that can be handled, and the larger the wire, the more is the cost. In addition, even for small wires this method produces a metal contact ridge which is lower than desired in some applications.

The present invention produces a relatively high ridge of valuable-metal contact strongly anchored, which employs an electrically determined minimum of the valuable metal. An incidental advantage is the use that can be made of certain of the old apparatus for welding plain platinum wire to backing materials.

A basic material used in the invention will first be described. ,This is a wire similar to Wollaston wire. A Wollaston wire is a very fine (usually valuable such as platinum) wire, ordinarily prepared by first incasing a wire core in a cylindric sheath of less valuable metal and drawing the whole down to a small size. Then the sheath is dissolved ofi, leaving the inner wire for ultimate use. While some Wollaston wire is made so small as to be useful as cross hairs in telescope eye pieces, etc., the present invention contemplates the use of somewhat larger sizes than these and wherein there is a metallic fused or welded bond between the core and sheath. These are now produced by known methods. But it is to be understood that the dimensions used are essentially small being variable to suit the size of contact desired. Hereinafter the organization of the bonded Wollaston type of wire in its sheath subsequent to drawing but prior to dissolving off the sheath will be called a composite bonded Wollaston wire. It is this that forms one rawmaterial element for the invention.

Referring now more particularly to Fig, 1, there is shown at numeral I a round composite bonded Wollaston wire prior to the time that its incasement is dissolved away. This consists in this case of a platinum core 3 with a more readily soluble sheath 5, preferably of steel in the present application. The assembly 3, 5 is then drawn down to a size, for instance .012 inch in outside diameter in which the core is about .006 inch in diameter. The sidewise periphery of the sheath 5 of this composite bonded Wollaston wire I is then welded as at 9 to an electrically conductive, pref-- erably brass, support I (when steel is used for the incasement 5) Electrical welding is preferable.

Next, the organized assembly shown in Fig. 1 is inserted into a suitable pickling bath which will remove the steel shell without attacking the platinum, or brass. A satisfactory material in the above example would be dilute sulfuric or hydrochloric acid. This pickling material will not attack the platinum 'wire-3 nor the non-ferrous support 1. It will be seen that the weld 9, being in a re-entrant pocket under the wire, will be much more slowly eaten away than the exposed upper and side arcuate surfaces of the a shell 5. The final appearance is diagrammed in Fig. 4 wherein only the platinum wire 3 is left with a small portion of the original steel incasement 5 beneath it which forms a wire-supporting member joining it to the backing I through the substantial remains of the weld 9. Electric contact welding at 9 involves no additional material at that region, but if fusing processes are resorted to requiring additional material, it should preferably, though not necessarily, be such as to resist attack entirely by the pickling fluid, The point in any event is that the upper surface of the wire 3 is entirely etched oif and exposed long before the wire 3 is disconnected from the support I, and it is a simple matter to time the pickling operation to cease when the wire 3 is exposed and before the disconnection occurs.

It will be understood that the choice'of materials and pickling baths is wide within the principles above set forth. For example, the bonded Wollaston type wire may have a platinum core 3 with a silver or copper alloy incasement, in which case a steel support 1 would be preferable in order that the pickling agent which attacks the incasement will not attack the base. The principle is to have a wire core and base which are not subject to attack by the pickling agent which will then selectively remove the upper part of the incasement from the wire.

For removing the sheath 5. if it is silver and the base is steel, then the commonly known anodic etching may also be used, in which case the contact to be etched away is made the positive pole in a cyanide solution.

In Fig. 2 a modification is shown in which a rectangular-drawn composite bonded Wollaston type wire ii is used. Hence there is a, rectangular core l3 and a flat walled incasement i5. This is attached to the backing base or support 11, by welding or the like I9, and then etched away as shown in Fig. 5. It will be understood that a rectangular cross section is useful for certain purposes and is more economical of valuable metal.

In Figs. 3 and 6 is shown a third modification in which elliptical composite bonded Wollaston type wire 2| is used having an elliptical core 23 with an elliptical incasement 25 held as at 29 to a base 21. This form is also more economical than that of Fig. 1.

It will be clear that the Fig. 2 and Fig. 3 forms of the invention employ more or less fiat Wollaston type wires, whereas Fig. 1 does not.

It is to be understood in connection with all of the forms of the invention thus far shown that the welding may be accomplished without additional metals if electric welding be resorted to, in which event the metals of the sheaths and the bases 1 will be interfused per se at point of contact, but any desired additional welding or brazing material may be used. The results will be similar to those above described, because with or without additional metal at the point of attachment between the base and the composite bonded Wollaston type wire, the etching operation is not very effective in a corner or screened position and therefore proceeds slower at such a point. The screening is provided by the support and 1core which sandwiches the supporting materia It will be noted from Figs. 4, 5 and 6, that there is a pyramidal outline at the point of attachment between the platinumwire and the base which provides a very strong support and relief of the core for use as an electrical contact. Thus, even though the core 3 is quite small, it may reach or be reached by the opposing contact (not shown). At the same time, the base 1, I l or 21, as the case may be, may be made as heavy as desired for strong and reliable organization with the remainder of the switch gear.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As many changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

I claim:

An electrical contact comprising a length of a fine wire of valuable, electrically conductive metal, said wire, in order to conserve said valuable metal, being of such small cross-section that it would be diificult to handle unless encased in a sheath, an electrically conductive base carrying said wire, the wire being mounted on said base in electrically conductive relation with respect thereto by means of an intermediate wire-supporting member of a non-valuable electrically conductive metal with the length of wire extendin substantially parallel to and spaced outward from the base, characterized in that said electrical contact is formed by welding the sidewise periphery of a sheath for the wire of a non-valuable electrically conductive metal to the base, and etching away all of the sheath except a portion intermediate the base and wire, said portion constituting said intermediate wire-supporting member, said base being formed of a metal which is resistant to etching by at least one etching agent which will etch the non-valuable metal of said sheath so that the base is not substantially etched away during the etching of said sheath. GEORGE DURST.

REFERENCES orrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 998,965 Gwozdz July 25, 1911 980,420 Hulfish Jan. 3, 1911 2,396,332 McBerty Mar. 12, 1946 

